1. Field of the Invention
This invention related generally to determining the quality of layers deposited on a substrate and in particular to measuring the thickness of a layer deposited on a substrate.
2. Description of Related Art
Use of automated techniques to process substrates, such as semiconductor wafers, is known in the art. Typically, a group of reactors, such as rapid thermal process reactors, are assembled in a cluster to perform a sequential set of steps in a substrate process flow. FIG. 1 is a top view of a cluster of three reactors 110, 120, and 130. Each reactor is used to perform a particular substrate process (e.g., deposition, annealing, etc.).
Reactors 110, 120, and 130 are arranged around sealed chamber 105 in which robot 104 is located. A plurality of substrate cassettes 102A, 102B, and 102C, each containing a plurality of substrates stacked on top of each other, is located in cassette room 103 adjacent clean room 101.
Substrate cassettes 102A, 102B, and 102C are first transferred from clean room 101 to cassette room 103. A computer control system directs robot 104 to take an appropriate wafer from a wafer cassette, e.g., wafer cassette 102A, from cassette room 103 and load it into an appropriate reaction chamber of a reactor, e.g., reactor 130.
Robot 104 also transfers wafers from the reaction chamber of one reactor 130 to the reaction chamber of another reactor 120. Consequently, the substrate process flow is quickly performed using robot 104 and a group of reactors, e.g., reactors 110, 120, and 130.
One problem encountered in a substrate process flow such as that illustrated in FIG. 1 is verifying that a layer deposited on the substrate is of acceptable quality. Presently, to determine the thickness of a layer deposited on the substrate, a variety of optical techniques are used.
Unfortunately, each of these techniques requires that a substrate be removed from the reactor and placed back in a cassette. The wafer is then removed from the cassette and taken to a machine, that measures the film thickness, in the clean room. This makes measuring the thickness cumbersome and relies upon the assumption that the thickness measured for a particular substrate is indicative of the thickness for a batch of substrates.
Typically, the optical techniques used include infrared reflectance measurement methods, interferometric methods, and ellipsometry. While these optical techniques are useful, each presently requires a measurement apparatus removed from the reactor cluster which in turn limits the throughput of the reactor cluster and complicates thickness measurements.